1. Field of the Invention
This invention relates generally to a dual-clad fiber laser and, more particularly, to a dual-clad fiber laser employing a dual-clad fiber array and a multi-mode pump array that are fused together with a doped silica frit so that a pump beam introduced into the pump array is able to propagate into the dual-clad array.
2. Discussion of the Related Art
Dual-clad fiber amplifiers or lasers are well known devices for providing high power optical beams in a variety of applications. The known dual-clad fiber lasers typically employ an array of fibers, where each fiber in the array includes a single-mode (SM) core, a multi-mode inner cladding layer formed around the core, and a thin outer cladding layer formed around the inner cladding layer. The inner cladding layer typically makes up the bulk of the fiber. The fibers are made of a suitable optical waveguide material, such as silica, where the core is doped with a material, such as ytterbium, that increases the index of refraction of the silica, the inner cladding layer is undoped silica, and the outer cladding layer is doped with a material, such as fluorine, that decreases the index of refraction of the silica. Therefore, the core has the highest index of refraction and the outer cladding layer has the lowest index of refraction. Thus, light propagating down the core is reflected off of the interface between the core and the inner cladding layer, and light propagating down the inner cladding layer is reflected off of the interface between the inner cladding layer and the outer cladding layer. Because the core has a higher index of refraction than the inner cladding layer, light in the inner cladding layer crosses the core as it propagates down the fiber.
A pump beam is introduced into the inner cladding layer through an end of each fiber at an incident angle greater than the critical angle, so that the pump beam is trapped by internal reflections in the inner cladding layer as it propagates down the fiber. A signal beam is introduced into the core through an end of each fiber at an angle greater than the critical incident angle so that the signal beam is trapped by internal reflections in the core as it propagates down the fiber. As the pump beam propagates down the fiber and is reflected off of the interface between the inner cladding layer and the outer cladding layer, it crosses the core. The atoms forming the laser medium, such as ytterbium, in the core absorb the pump light, which increases the internal energy of the atoms. The increased energy of the laser atoms within the core is transferred to the signal beam by stimulated emission so that the signal beam is amplified. The specific physics of the transfer of energy from the pump light to the signal light in a dual-clad fiber laser of the type being described herein is well understood to those skilled in the art.
The diameter and number of fibers in a dual-clad fiber array depends on the amount of power an individual fiber can provide and the amount of optical power desired. In one example, the core of each fiber has a diameter of 10 microns, the inner cladding layer has a diameter of 200-400 microns, and the outer cladding layer has a thickness of 10-20 microns. Further, the wavelength of light used for the pump light and the signal light is also application specific. In one example, the pump light has a wavelength of about 920-950 nm and the signal light has a wavelength of about 1060-1090 nm.
The amount of signal beam amplification in each fiber is proportional to the amount of pump light absorbed in the core. For high power applications, it is necessary that the pump light be efficiently coupled into the core along the length of the dual-clad fibers. The known techniques for coupling the pump light into the inner cladding layers generally require a separate optical source for introducing the pump beam into an end of each fiber. This process is typically labor-intensive, and therefore usually unsuitable for high power applications. These techniques can be improved upon to provide an alignment-free, complete in-fiber and massively parallel process for coupling the pump beam into the core.
Improvements have been made in the art for coupling pump light into the inner cladding layer of a fiber associated with a dual-clad fiber laser. In one design, a single-mode fiber is surrounded by a plurality of multi-mode fibers that are similar to the dual-clad fiber, but with the single mode core removed. This bundle of fibers is heated and stretched so that the bundle is tapered to a smaller diameter. The tapered bundle is then fusion spliced to another dual-clad fiber so that the plurality of tapered multiple mode fibers align with the inner cladding layer of the other dual-clad fiber, and the core of the single-mode fiber at the center of the tapered bundle is aligned with the single mode core of the dual-clad fiber. Pump light coupled into the ends of the multi-mode fibers opposite to the dual-clad fiber propagates down the multi-mode fibers and into the inner cladding layer of the dual-clad fiber. Optical amplification of the signal beam is then provided in the core of the dual-clad fiber. U.S. Pat. No. 5,864,644 issued to DiGiovanni et al. discloses a cladding-pump fiber device of this type.
Although the above-described technique is effective for coupling pump light into an inner cladding of a dual-clad fiber, improvements can be made. What is needed is an improved technique for coupling pump light into a dual-clad fiber laser or amplifier array, that is alignment-free, completely in-fiber and massively parallel. It is therefore an object of the present invention to provide such a laser or amplifier.
In accordance with the teachings of the present invention, a dual-clad fiber laser is disclosed that employs a multi-mode pump fiber array for introducing pump light into a dual-clad fiber array along the length of the array. Each fiber in the dual-clad fiber array includes a single mode core, a multi-mode inner cladding layer and an outer cladding layer. Each fiber in the multi-mode fiber array includes an inner portion and an outer cladding layer. The inner cladding layer and the inner portion are the same material, and the outer cladding layers of the fiber array and the pump array are the same material. In one embodiment, the dual-clad fiber array is a fiber ribbon that is wound on a bobbin, and the multi-mode pump fiber array is a fiber ribbon that is wrapped around the outside of the dual-clad fiber ribbon.
A doped silica frit is placed between the pump ribbon and the fiber ribbon, where the dopant makes the frit have an index of refraction that is greater than the index of refraction of the inner portion and the inner cladding layer and therefore also greater than the index of refraction of the outer cladding layers. The assembly is heated so that the silica is softened. The silica frit, a portion of the outer cladding layer and the inner cladding layer of the fibers in the dual-clad fiber ribbon, and a portion of the cladding layer and the multimode core of the pump ribbon fuse together so that the index of refraction of the fused material is about the same as silica. Therefore, the outer cladding barrier where the dual-clad fibers and the pump fibers meet is removed, and pump light that is introduced into any of the fibers of the pump ribbon is free to cross into the inner cladding layer of all of the fibers in the dual-clad ribbon. The remaining portion of the outer cladding layers provides an outer optical barrier for containing the pump light in the fused structure.
Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims taken in conjunction with the accompanying drawings.